A human monocyte-like cell line, U937, can be stimulated to differentiate into macrophages in vitro in the presence of lymphokines. These U937 cells do not kill in vitro spontaneously even after activation. However, when co-cultured for 7 days with normal human lymphocytes these U937 cells will show cytotoxic function. It is felt that a helper lymphocyte may be necessary in the development of monocyte cytotoxicity and it is hoped that through studying these and other monocyte cell lines that the mechanism of monocyte activation can be more clearly defined.